The present technology generally relates to systems and methods for flash-evaporating black liquor and other liquids to increase the concentration of desirable solutes in a solvent. More particularly, the present technology relates to a pressurized vessel (“flash tank”) for flash-evaporating such material.
Flash-evaporation occurs when a saturated liquid stream undergoes a rapid reduction in pressure. If the saturated liquid stream is a solution of various liquid chemicals, the reduced pressure causes chemicals with high volatility to evaporate rapidly out of the saturated liquid solution. The portion of the solution that remains in liquid form (also known as flashed liquid or flashed liquor) will invariably have an increased concentration of liquids with lower volatilities. These can be desirable solutions in many industrial processes. Flash tanks typically feature an inlet nozzle connected near the top of the flash tank. They may also have an exit port located at or near the bottom of the flash tank. Flashed liquid that remains after the flash evaporation may exit through this exit port.
Industrial flash tanks are generally used to flash-evaporate a high pressure liquid stream to produce a steam stream and a flashed liquid stream. These flash tanks typically have a high pressure inlet nozzle that communicates the high pressure liquid stream to the interior of the tank. They also typically feature an upper steam recovery system with a gas discharge port, and a liquid discharge port. Steam recovery systems may employ additional components. Flash tanks safely and efficiently reduce pressure in a pressurized liquid stream, thereby allowing recovery of heat energy (steam) from the flashed liquid stream. They may also be used to concentrate chemicals in the flashed liquid stream.
In practice, a high pressure liquid stream usually flows through the inlet nozzle and is either sprayed against a deflector plate of various shapes or along the wall of the flash tank. The percentage of volatile chemicals that flash-evaporate from the high pressure liquid stream increases upon exposure of the chemicals to the low pressure environment. As such, many conventional flash tanks utilize inlet nozzles to spray the incoming high pressure liquid stream in a uniform direction along the inner flash tank walls to increase the incoming high pressure liquid stream's exposure to the low pressure environment as it spirals downward toward the level of flashed (condensed) liquid. Consequently, the flashed liquids at the bottom of the flash tank tend to spin in a uniform direction. A vortex breaker is usually employed to disrupt this spinning at the bottom of the flash tank to facilitate the exit of flashed liquid from the flash tank.
One problem with large scale flash-evaporation equipment is that traditional inlet nozzles force the incoming high pressure liquid stream to converge to a point as they eject the high pressure liquid stream along the inner vessel wall. The resulting collision of the high pressure liquid stream with the inner flash tank wall causes disruption in the formation of the uniform flow on the inner chamber wall, thus reducing the amount of volatile liquid extracted from the high pressure liquid stream.
Large scale flash tanks suffer from another problem: a small portion of desirable low-volatility chemicals may condense around high volatility chemicals in the steam. In industrial processes, this can lead to a significant loss in desirable product, increased operating costs, and increased release of harmful chemicals into the environment. As such, many industrial flash tanks feature steam recovery systems to process or repurpose the steam. This steam may be utilized as heat energy in other stages of the process, or it may be discharged in the appropriate manner.
Flash tanks are common pieces of equipment in many chemical industrial processes. They can be used in batch or continuous chemical manufacturing processes. Pulp and paper production and biomass treatment are typical industrial processes utilizing one or more flash tanks to recover steam from hot high pressure liquid process streams produced by treating comminuted cellulosic fibrous material, lignocellulose, or other such material.
Flash tanks may be used to recover chemicals from chemical pulping systems, such as sulfur, soda, or Kraft cooking systems. To produce pulp from wood chips or other comminuted cellulosic fibrous organic material (collectively referred to herein as “cellulosic material”), the cellulosic material is mixed with liquors, e.g., water and cooking chemicals, and transferred to a pressurized treatment vessel (“digester”). Sodium hydroxide, sodium sulfite, and other alkaline chemicals are used to “cook” the cellulosic material in a Kraft cooking process. Other cooking processes, for example, the soda cooking process, may use alkaline chemicals free of sulfur.
These cooking chemicals and many combinations thereof are known in the pulp and paper industry as white liquor. As the white liquor contacts the cellulosic material, it begins to degrade lignin, hemicellulose and other compounds in the cellulosic material. The white liquor quickly incorporates dissolved organic compounds and becomes black in color and may be referred to as “black liquor” or even “spent cooking liquor”. As such, spent cooking liquor is commonly referred to as “black liquor” in the industry. The Kraft cooking process is typically performed at temperatures in a range of 110° C. to 180° C. and at pressures substantially greater than atmospheric. The soda cooking process may be preformed at higher temperatures and pressures than the Kraft cooking process.
Cooking digesters may be batch or continuous flow vessels. They are generally vertically oriented and may be sufficiently large to process 1,000 tons or more of cellulosic material per day, wherein the material remains in the vessel for several hours. In addition to a Kraft, soda, or sulfur digester, a conventional pulping system may include other pressurized reactor vessels for impregnating the cellulosic material with white liquor, or black liquor, prior to the cooking in a digester. In view of the large amount of cellulosic material in the impregnation and cooking stages, a large volume of black liquor tends to be extracted from these pressurized reactor vessels.
The black liquor includes the cooking chemicals (such as residual alkali) and organic chemicals (such as organic acids), as well as dissolved organic materials e.g. lignin, hemicellulose, and other organic materials dissolved from the cellulosic materials. Removing some of the black liquor containing a high volume of dissolved organic materials at various stages of the pulping process has been found to increase various pulp properties including tensile strength. This has been disclosed in U.S. Pat. No. 5,489,363. In the pulping process, flash tanks are used to produce steam from hot process liquids, hot high pressure liquid streams, such as black liquor which results in concentrating the dissolved organic material in the resulting flashed black liquor (may also be referred to as concentrated black liquor). The flashed black liquor leaving the flash tank is at a lower pressure than the hot high pressure liquid stream entering the flash tank. This flashed and concentrated black liquor can be used for further processing, such as in the evaporation and recovery parts of the mill where chemicals are recovered and dissolved solids can be used as a fuel to create energy, or for use in another stage of the pulping process.
The black liquor is flash-evaporated in a flash tank to generate steam and flashed liquid. The cooking chemicals and organic compounds are included with the flashed liquid formed when the black liquor is flashed. The steam formed from flash-evaporation is generally free of condensable chemicals and organic compounds, but could contain non-condensable gas such as hydrogen sulfide, etc. Steam produced by flash-evaporation of the high pressure liquid stream from the pulping process may be used as heat energy in the pulping process, that is, returned to the pulping process as heat energy.
In conventional flash tanks with an integral steam chamber, a portion of the steam chamber is substantially engaged with the circumference of the flash tank. The remainder of the steam chamber tends to be recessed, thereby creating a cavity above the interior chamber. This cavity has been used to reclaim condensable liquids such as black liquor for reuse in the cooking process; however the fact that the steam chamber is substantially engaged to the circumference of the flash tank reduces the surface area along which the high pressure liquid stream may travel down into the flashed liquid below.
The interior of the steam chamber usually contains a series of baffles designed to create a tortuous path for the exiting steam and thereby reduce loss of condensable liquor. As steam passes through a convoluted internal path, the corrosive nature of the black liquor and the high pressures contained within the flash tank causes damage to the tank or causes deposits on the interior of the tank, thereby requiring periodic maintenance to repair and clean the flash tank. As such, the extent to which baffles could extend into internal chambers of the steam chamber is limited by the need to make all areas of the steam chambers wide enough for human admittance. In order to meet the requirement of the steam chambers being wide enough for human admittance, the steam chambers are thereby prevented from extending the baffles to be overlapping within the internal chambers of the steam chamber and thus limiting the surface area of the tortuous path for the exiting steam and thereby allowing for the loss of condensable liquor to exit with the steam.
Accordingly, there is a need for an improved steam chamber that will improve the condensable liquid recovery in the steam chamber without requiring admittance of a person for manual inspection. It is to these and other needs that the present technology is directed.
Conventional flash tanks also generally have inverted conical bottoms. These bottoms facilitate rotational movement of the flashed black liquor and also limit the surface area of the flash tank wall that can be used for conveying flashed black liquor or other flashed liquids to the liquid at the bottom of the flash tank. Traditional conical bottoms may also employ a vortex breaker to disrupt the rotational movement of the flashed black liquor before allowing it to exit through a discharge port at or near the bottom of the flash tank. Accordingly, there is a need for an improved design that will increase the surface area of the flash tank's interior wall without disrupting the continuous flow of flashed black liquor out of the flash tank.